The word 'mong' only came about because of the racist nomenclature that said "Hey these retards look mongolian. Mongolians look like retards. Let's call retards 'mongs'"
This is an interesting point, to be honest. The original use of the word "mongoloid" to describe someone suffering from down's syndrome does indeed seem to have racist origins. That said, very rarely is the word used in a racist context. An offensive context, sure, but very rarely racist.